The striate cortex stands out among the different areas of the early visual syste chiefly because of the unique receptive field properties of its neurons. Intricate synaptic connections between cortical neurons and geniculate afferents, and amount cortical neurons themselves, produce essential changes in the form in which the visual image is encoded, a change that is of interest not only in its own right, but as a model for functions throughout the neocortex. An explanation of cortical receptive fields must include a description of synaptic inputs onto individual cells: their sources, their receptive field properties and how they interact. The experiments in this proposal are designed to provide such a description by examining synaptic potentials directly with intracellular recording techniqes. Electrical stimulation will be used to reveal the sources of synaptic inputs; visual stimulation will be used to characterize their visual response properties. The largest effort will be devoted to studying the origins of orientation selectivity in cortical neurons, in particular to determining whether the spatial organization of thalamic input is by itself sufficient to construct this unique cortical property, or whether intracortical inhibitory connections make a substantial contribution. The major experiment will be to record carefully the orientation selectivity of visually evoked IPSPs and EPSPs in cortical cells. Various methods will be used to insure that all synaptic potentials present in the cell are detected. In addition, analysis of recorded potentials will make it possible to determine the receptive field properties of individual presynaptic inputs. By performing the same experiment in the presence of a drug that inactivates ON-center cells of the lateral genieulate nucleus (APB), it will also be possible to determine how the interactions between On-center and OFF-center cells contributes to orientation selectivity. Finally, the interaction of X and Y cells of the LGN will be studied by activating them separately with an electrical stimulus in the optic nerve.